Is Z-Gaz billing legit?

The last time I had my 20lb propane tank filled at Z-Gaz (on the carreterra across from Salvador's restaurant), the pump guy directed me to the office to pay for the filling while he was still filling the tank. Inside the office, the guy handed me a pre-printed bill which I paid for without much thought. When I returned to the pump, the guy was very helpfully putting the tank in my car. I drove away happy with the service.

However, that tank of gas lasted about half as long as previous fills. Thinking I had either left the valve open or there was a leak in the tank or I had absent-mindedly used a lot more gas, I went back for a refill. While there, I had the pump guy inspect the tank and valve assembly. He pronounced them OK; I went into the office and paid the pre-printed bill as I previously had. That fill-up lasted me about 1/2 as long as well. I think I see the problem here.

It's time for a refill once again. I am just wondering if I should go elsewhere, or, has the filling/billing discrepancy problem been corrected at Z-Gaz?

If a business burned me twice, I certainly wouldn't go back again -- ever.

I've only had good luck with Z gas. But they were delivering to my house and I checked the % full gauge both before and after they fill the 300 liter tank.

Would it be possible to pay first and then watch them filling the tank? And is there a way to check a gauge to see how full your small tank is? Or could you weigh the tank both before and after it was filled?

The reason I bring this up is that I don't normally hear bad things about Zeta Gas. It was my understanding, at least in my Ajijic neighborhood that they were honest. (We all check how much was put in our tanks. I hope that was not the only reason they were honest)

Weigh the tank before and after. A fill should be what it is called - 20 lbs. heavier (if it was empty when you took it in to be filled).

I seems that they may be charging a flat fee for those small tanks; hence the suggestion to just go inside and pay a pre=printed bill.
That would make some sense, since the amount is a drop in the bucket compared to the 30L or 300l tanks, which are much more common and no more labor.

Yes, if you notice, they place the tank on a scale while they are filling it so they know how many lbs. (or kgs.) they are putting in.

I had the "go pay while I fill" experience at the Z-Gas station east of Chapala a few months ago; when I used the Ajijic place in November, I saw no scale, but I was charged according to what the pump showed. I don't remember if the pump shutoff automagically -- e.g., sensed back pressure? -- or if the filler just "knew."

On your lp tank you have a tare weight or empty weight. In the old days that plus 20 lbs was a full tank at 80%. You need a vapor chamber so the liquid has a place to become a gas. In the US the new standard is 17lbs. for safety and max profit. The claim is the new safety valve is full at that point. Weigh your tank now and subtract the tare weight to see how much gas is in the tank. My experience is I get over filled if any gas is in the tank as they fill by number of liters.
WC on the tank is water capacity in gallons usually. Divide by 3.8 the liters in a gallon and you know how many liters the tank holds. See if that number is close to your receipt.

Many thanks for the replies and suggestions. I guess I will have to get a scale if I want to pursue this.